At present, it is required to adopt an absorption spectrometry method to detect micro-samples with the concentration levels of ppm and ppb in the application of many fields. In order to improve the detection sensitivity of the absorption spectrum technology for a low-concentration sample, lengthening an optical path of light beams passing through a sample is an effective method. Obviously, if it merely keeps a light source away from a detector to allow the light beams to pass through a very long and straight transmission-type sample measurement pool, shortcomings of bulky devices, complex collimation and poor temperature stability and anti-vibration performance will be caused. In general, a “folded” optical path is adopted. That is, the optical path is repeatedly reflected between reflecting mirrors to lengthen the effective optical path in a smaller spatial region. For example, White sample pools and Herriott sample pools are the more common ones. Each of them adopts a concave spherical reflecting mirror to repeatedly reflect the optical path in the smaller spatial region. Likewise, there is also a way of adopting a plane reflecting mirror. For instance, a U.S. Pat. No. 3,524,066 describes a technical solution that two plane reflecting mirrors are mounted at two ends of a cylindrical cavity to achieve multiple reflections of the optical path.
However, in practical use, if a test environment is poor (due to excessive dust, corrosive substances, high humidity and temperature, vibration, etc.), in a reflecting mirror-based sample measurement pool, as the front surface is plated with a high-reflecting film (that is, a reflecting film is plated outside the mirror), a reflecting surface and a contact surface in contact with a sample of a reflecting mirror are overlapped; that is, they are the same surface. In practice, the high-reflecting film is in direct contact with the sample in the sample pool, such that the film is likely to be damaged from the sample and impurities therein.
In general, the high-reflecting film is a metal reflecting film or a dielectric reflecting film. If it is the former, the high-reflecting film is very likely to be scratched due to lower metal film hardness of the single-layer metal film. Thus, it is often required to plate the metal film with a protective film. Therefore, the obtained metal reflecting film typically comprises multiple films as the metal reflecting film itself is made via several times of plating. Similarly, if it is the dielectric reflecting film, the obtained dielectric reflecting film also comprises multiple films as the dielectric reflecting film itself is typically made via several times of plating. No matter it is the metal reflecting film or the dielectric reflecting film, when being located in a poor environment with high temperature and humidity, excessive dust, corrosive substance, etc., the high-reflecting film is likely to be damaged since gas, water vapor and impurities in the environment are easy to enter into spaces between every two films of the high-reflecting film and a junction of the high-reflecting film and a glass substrate arising out of limited adhesion between every two films and between the film and a substrate material, and compactness of the films. Moreover, in a preparation process of the high-reflecting film, granular foreign matter is always introduced, resulting in defects in the high-reflecting film. If the reflecting mirror whose film is defective is located in the poor environment, a damaging effect will be exacerbated. As a result, not only are the reflectivity of the high-reflecting film reduced and light energy wasted, but also a film peeling off in a damaged process may block the optical path, further reducing the light energy collection efficiency. What's worse, the glass is deformed and broken under a stress effect of the high-reflecting film, resulting in poor environmental adaptability of the sample measurement pool based on the film-plated reflecting mirror, so that the sample measurement pool cannot adapt to different measurement environments or cannot be normally used in the poor environments.